The speed measurement and length measurement of a moving object are greatly demanded in the production and life Radar speed measurement instruments have limited service life and limited accuracy. In addition, the laser speed measurement is high in cost. The length measurement of a moving article on a production line is an important means for quality control and product grading. Further, the length measurement is the basis of size measurement and accurate measurement and control. However, the measurement of a moving object is a challenging technical problem. Low-cost measurement means should be a direction of human pursuit, and its development and applications will certainly facilitate the development of related technologies and promote the living quality and production level of people.
Moreover, the measurement of running vehicles is an important problem in the traffic control and safety check. During various types of scanning to vehicles, particularly during X-ray scanning, the measurement and control on running vehicles are highly required. The use of length measurement and speed measurement means having a high cost performance is a key basis of the measurement and control.
For a top irradiation type X-ray check system that is developed recently, to avoid the effects of a dragging device on a scan image, relay type dragging is introduced (that is, front and rear dragging devices are respectively used for dragging) so as to achieve steady and seamless relay type dragging. However, how to realize accurate measurement and control is also a key of affecting the quality of a scan image.